1. Field of the Invention
Aspects of the present invention relate to a lithium secondary battery, and more particularly, to a separator of an electrode assembly of a lithium secondary battery.
2. Description of the Related Art
A secondary battery has many advantages such as rechargeability, a compact size, and a high capacity. Recently, various kinds of secondary batteries have been being widely researched and developed as power supplies for portable electronic devices such as camcorders, portable computers, and mobile phone. Representative secondary batteries that are being currently developed include nickel hydrogen batteries, nickel metal hydride (Ni-MH) batteries, lithium ion batteries, lithium ion polymer batteries, and the like.
Since lithium, which is widely used as a material of a secondary battery, has a small atomic weight, it is an advantageous material to use for obtaining a battery having a large electric capacity per unit mass. On the other hand, since lithium violently reacts with water, it is typically necessary to use a non-hydrophilic electrolyte in a lithium-based battery. Advantageously, such a lithium secondary battery is capable of generating an electromotive force of about 3 to 4V because it is not influenced by the electrolysis voltage of water.
Non-hydrophilic electrolytes used in lithium ion secondary batteries are classified into liquid electrolytes and solid electrolytes. Liquid electrolytes are formed by dissociating lithium salt into an organic solvent. The organic solvent may often include ethylene carbonate, propylene carbonate, carbonate having other alkyl group materials, or equivalent organic compounds.
Solid electrolytes are capable of carrying ions such as a lithium ion when a voltage is applied. Solid electrolytes can be classified into organic group electrolytes composed of polymers and inorganic group electrolytes composed of crystalline or amorphous inorganic materials. Solid electrolytes are considered to overcome problems of liquid electrolytes, such as freezing in a low temperature, evaporation in a high temperature, and leakage.
A non-hydrophilic electrolyte typically has low ion-conductivity, and particularly, the ion-conductivity of solid electrolyte are typically lower than that of liquid electrolytes. The lower ion-conductivity of solid electrolytes may be compensated for by increasing the area of an activation material in the electrodes and enlarging the facing areas of the electrodes.
However, the enlargement of the facing areas of the electrodes is limited by various constraints. Consequently, the low ion-conductivity of the electrolyte increases internal battery impedance, thereby resulting in a significant internal voltage drop. Particularly, when high current discharge is necessary, the low ion-conductivity of the electrolyte limits the current in the battery and the battery output. Therefore, it is desirable to continuously make efforts to improve the ion-conductivity of these types of batteries.
The separator may also serve as limitation on the mobility of lithium ions between the two electrodes. Except for the case in which a pure solid electrolyte also functions as a separator, when the separator between the two electrodes does not have sufficient permeability or wettability for the electrolyte, the limited mobility of lithium ions accordingly creates difficulty during discharge. In the solid electrolyte separator, the thickness of the separator corresponding to the ion path is an important parameter of internal resistance.
Important characteristics of a separator relating to performance of a battery are ion-conductivity (of a polymer separator), thermal resistance, resistance to thermal deformation, chemical resistance, mechanical strength, cross-sectional porosity (representing a percentage of the area of pores in a particular cross-section of the separator), wettability for an electrolyte, and the like.
On the other hand, a separator of a lithium secondary battery using a liquid electrolyte also naturally functions as a safety device for preventing overheating in the battery. It is known that a microporous film made of a polyolefin or its equivalent as a typical material of a separator may be softened and partially melted when its temperature increases over a predetermined level in an abnormal condition. Accordingly, pores of the microporous film, functioning as a path for the electrolyte and lithium ions, are shut down in an abnormal condition so that transportation of lithium ions is blocked and the internal and external current flow in the battery stops. Therefore, it is possible to stop the temperature increase of a battery.
However, when the temperature of a battery abruptly increases for reasons such as external heat transfer, the temperature of the battery may remain high for an extended time period in spite of the shutting down of the pores of the separator, and the separator may be damaged. Specifically, an internal short circuit may be generated when the separator shrinks or is partially melted, and facing portions of the electrodes may make contact with each other in the shrunken or melted area. This type of short circuit is considered to be very dangerous.
Recently a high capacity secondary battery has been manufactured to provide a large current flow over a short time period. In this type of battery, once an abnormal overcurrent condition occurs, the temperature of the battery is not immediately reduced, but the separator is continuously melted by the existing heat, even when the pores of the separator are blocked. Accordingly, the possibility of generating an internal short circuit caused by damage in the separator increases even higher.
Considering the aforementioned scenarios, the shrinkage or melting of the separator in an abnormal overheating condition is a more critical issue than the cutoff of the current by blocking of the pores of the separator. Consequently, it is desirable to provide a secondary battery that can safely prevent a short circuit between the electrodes at a relatively high temperature, such as, for example, 200° C. or more.